According to the American Heart Association, approximately 615,000 stent procedures are performed each year in the United States to treat coronary artery disease. The stents used in these procedures fall into two major categories: bare metal stents and drug-eluting stents. Traditional bare metal stents are commonly made from 316L stainless steel, cobalt-based alloys, tantalum, or titanium alloys such as nitinol. These stents are effective in opening up the narrowed arteries in the short-term, however they suffer from high rates of restenosis, with about 30% of patients requiring additional invasive follow-up procedures within 3-6 months of implantation. In the case of drug-eluting stents, the stent surface is coated with a polymer (e.g. polyethylene-co-vinyl acetate (PEVA) and poly n-butyl methacrylate (PBMA)). Cytostatic drugs such as sirolimus or paclitaxel are formulated in the polymer coating such that they can be released though the healing period. These drugs are used as anti-proliferative agents to prevent proliferation of SMCs.
Several large clinical studies have demonstrated lower restenosis rates in patients treated with drug-eluting stents when compared to bare metal stents. However, recent studies suggest that drug-eluting stents may increase the risk of late thrombosis leading to increased risk of patient stroke or heart attack. It is hypothesized that degradation of drug-eluting polymer coating leads to incomplete re-endothelialization of vessel lumen. Further, this can also induce inflammation that can activate phagocytic lymphocytes. This inflammation may initiate the clotting cascade, which is one cause of vascular thrombosis. About 20% of the patients with drug-eluting stents suffer from restenosis followed by late thrombosis leading to stent failure. Thus while bare metal stents and drug-eluting stents are effective in treating coronary artery diseases, the existing technologies have several major drawbacks in terms of long-term success. A need remains for improved implantable devices and methods for surface modification of devices for improved performance in vivo.